The present invention relates to an apparatus for evaluating, by way of measurements, optoelectronically-produced raster image in which, for the purpose of regulating the movement of a tape- or band-shaped material (hereinafter called a tape), the position of the tape is detected, with the aid of a video camera, by continuously reproducing one edge of the tape, or a marking line on the tape, as a sudden change in contrast. More particularly, the present invention relates to such an apparatus wherein, for the purpose of evaluation, each picture raster or frame is scanned line-by-line and the picture elements, e.g., dots, in each line are counted by means of a picture element counter until the selected change in contrast occurs and then the counting result is converted, by means of a digital/analog converter, into an analog output signal from which an adjustment order is formed which mechanically brings the tape position into the desired position.
Known tape movement regulators are closed control circuits in which measuring members measure, usually without contact, the momentary position (actual position) of the edges of the panel of the band- or tape-shaped material to be transported and thus detect its position. Photoelectric measuring barriers, inductive measurement sensors or pneumatic sensors are used for this purpose.
The periodical "VDI-Nachrichten" No. 23. 1980, page 6, also discloses cases in which the tape edge and tape position are monitored on a monitor with the aid of a television or video camera and simultaneously a video signal is generated which measures and detects the position of the tape edge. Such uses are applicable, for example, for steel and stainless steel tapes which pass at high speed through a heat-treatment furnace wherein tape temperatures of more than 1000.degree. C. are realized and a large quantity of tape can be supplied. Such uses require special techniques of contactless measurements on the tape edges of the purpose of guiding the tape in a centered position. These measurements are made with the aid of two industrial use television cameras which are mounted in symmetry with the center of the system and operate through a heat-resistant observation window, with the cameras being water-cooled while their cable inputs and their front optical systems, as well as the illumination devices required for the detection, are air-cooled.
The problem in optically monitoring the tape position is to assure sufficient bright-dark contrast (sudden change in contrast) at the tape edges over the entire temperature range from about 20.degree. C. to more than 1000.degree. C. attained by the tape-shaped material (tape), as well as in the interior of the furnace, which is realized with measures of the above-mentioned special technique, inter alia, by extraneous illumination of the tape in the viewing field of the camera.
In a video camera, the optical picture content projected in the image converter plane by the front optical system is divided into a picture element raster which is electronically scanned successively line-by-line and results in an optoelectronically generated picture raster or frame. At the same time and in synchronism with the line-by-line scanning, the image content is reproduced on the associated monitor.
The optical detection of the tape position with the aid of video cameras and the evaluation of the optoelectronically-generated picture raster can be effected, in principle, in two ways;
1. With the aid of a video camera, the moving tape is covered or observed over an area extending beyond the entire width of the tape, i.e., both tape edges are covered. From the resulting picture or frame, an electronic output signal is formed on the basis of the evaluation of the positions of the two tape edges and this signal is proportional to the center position of the tape.
2. With the aid of two video cameras, which are arranged symmetrically to the center of the system, the two edges of the moving tape are covered or observed separately and the two pictures or frames produced optoelectronically in this way are each evaluated during their generation by means of a device which includes the above-described features.
The horizontal scanning of the picture content of the two cameras is done in opposite directions. The two electrical output signals obtained from the evaluation are both proportional to the distance between the edge position and the beginning of the measuring range and the difference between these two signals is proportional to the center line of the tape. In each picture line, the number of picture elements from the beginning of a line to the sudden change in contrast is counted during the scanning and is converted, by means of a digital/analog converter, into an analog signal, or the position of the sudden change in contrast is determined by integration to provide an analog signal.
However, it is possible, particularly in continuous flow heat-treatment furnaces, for weak light-dark contrasts or color contrasts to appear at the edges of the tape which are no longer detected as such in the image converter of the video camera, so that error measurements result which produce errors in the evaluation. If, for example, no sudden change in contrast is detected in some lines of the pictures or frames, then the output signal of the picture evaluation during this period corresponds to the evaluation that the field of view of the camera and including the tape is completely covered or that the tape is presently not visible or not present at all. Moreover, if sudden changes in contrast appear in some lines of the pictures at the wrong positions of the tape due to soiling or as a result of false flashes, the output signal during this period corresponds to values which indicate that during the time in question, the tape edge is not moving, i.e., part of the tape is damaged, or, if the reason is a longer stretch of soiling, that the movement of the tape is shifted in the viewing range.
For these reasons, the known evaluation devices exhibiting the above-mentioned features and operating as described above are not suited to differentiate between the above-mentioned events or malfunctions and actual interference with the movement of the tape, i.e., they are unreliable.